Traditionally, electric power bushings have been made of an electric insulating part formed by a thermosetting resin, such as epoxy, moulded around an electrical conductor. The purpose of the insulation has been to prevent electric discharges between the conductor and the wall of a container through which the bushing protrudes. The thermosetting resin has been provided with a sufficiently high thickness to provide a satisfactory functionality in this respect. However, moulding thick solid bodies of thermosetting material is a costly process, and alternatives have been searched for.
U.S. Pat. No. 4,458,101 discloses a gas-insulated bushing comprising an elongated epoxy insulating shell. An inner conductor at high voltage is disposed within the shell, and an electrically insulating gas is provided in a space between the conductor and the interior face of the shell.
DE-A1-10 2007 007 498 describes a grommet having a housing of metal in which conductors are disposed. The conductors are electrically insulated from the housing by a glass body provided in a space between the conductors and the housing.
U.S. Pat. No. 3,178,505 discloses a terminal bushing with an insulating outer shell within which a conductor stud is disposed. In the space between the conductor stud and the outer shell is an insulating gas or liquid, e.g. air or oil, provided.
WO-A1-2008/074166 describes a bushing and a method for producing the same by means of a mould. The bushing has a duct for accommodating a conductor rod and comprises an insulating shell which encapsulates an electrical field grading insulation which surrounds the duct.
Instead of using a thermosetting material for the bushing, thermoplastic materials have been suggested. In U.S.—Patent Publication No. 2006/0178026 there is described an electrical connector and a method for forming the same by means of a mould, where an interface shell is formed from a thermoplastic. In EP-A2-1 496 576 a connector comprises an insulating part and an electrically conductive contact part, where the insulating part is injection molded in a thermoplastic plastic.
In prior art, it has also been suggested to let the bushing comprise a tubular body made of a thermoplastic material which is connected to the conductor and has a thin-walled outer shell which is to be connected to the wall of a gas-tight container. There is an empty space between the conductor and the outer shell of the tubular body, and the empty space is in communication with the interior of the gas-tight container, which is suggested to be filled with an electrically insulating gas, such as sulphur hexafluoride, SF6. The use of thin walled bushings results in lower production costs in relation to the use thermosetting bushings of prior art.
EP-A2-0 381 638 discloses an electric conductor bushing for use in distributing plants which are insulated by the aid of insulating gas. The bushing has a tubular insulating body made of a thermoplastic and adapted to pass through a hole in the plant wall. A conductor extends coaxially through the insulating body, and the space between the conductor and the insulating body is filled with an insulating gas, e.g. SF6.
WO-A1-2007/065912 discloses a grommet having an insulting body which is made of a thermoplastic and comprises an outer shell and an inner sleeve which encloses a high voltage conducting element disposed in said sleeve along the entire length of the part of the conducting element which extends through said body. The space formed between the inner sleeve and the outer shell is filled with an insulating gas.
However, the use of insulating gas alone in the space between the conductor and the outer shell might be insufficient to attain a guaranteed prevention of electric discharges emanating from the conductor. Should there be any leakage of the insulating gas from the container, a device as suggested in WO-A1-2007/065912, e.g., becomes sensitive and is likely to be subjected to electric discharges. A disadvantage of thin walls of a thermoplastic might also be that they may be subjected to diffusion of moisture from the atmosphere into the container to which the bushing is connected. To overcome these problems, it has been suggested to substitute the gas in the space between the conductor and the thermoplastic outer shell for other materials.
EP-A1-1 845 596 describes a device for electric connection to an energy supply conductor for medium and high voltage comprising an insulating part which is made of a thermoplastic polymer and includes an outer shell and an inner sleeve which encloses a conducting element disposed in said sleeve along the entire length of that part of the conducting element which extends through said insulating part. The space formed between the sleeve and the outer shell is suggested to be filled with a solid material, such as silicone rubber.
WO-A2-2009/047357 discloses a device for electric connection to an energy supply conductor for medium and high voltage comprising an insulating part which is made of a thermoplastic polymer and includes an outer shell and an inner sleeve which tightly encloses a conducting element disposed therein. The inner sleeve is connected to the outer shell by radial walls or struts which axially extend along the axial extension of the conducting element. The space between the sleeve and the outer shell is filled with a filler of an electrically insulating material other than that of the outer shell. It is suggested that the filler comprises an elastomer, or polyurethane as a main constituent. A method for producing such a device is also disclosed, which includes the step of filling said space with a filler in a liquid state and permitting the filler to solidify.
However, there is still a need for an improved bushing for medium and high voltage which is less complicated to produce in relation to the above-mentioned prior art solutions.